Pitch controlled wind turbines are often operated at optimal tip speed ratio, i.e. in such a manner that, at each wind speed, an optimal tip speed ratio (λ) is obtained. To this end optimal tip speed ratio curves and optimal pitch curves are generated based on knowledge about expected optimal settings for obtaining an optimal power production. The curves may be generated purely on the basis of calculations. However, they may alternatively or additionally be based on measurements performed on identical or similar wind turbines. Some inaccuracies are introduced in the generation of the curves, because the numerical models used for calculating the values for the curves may be inaccurate, and because optimal settings may vary from one wind turbine to another, from one site to another and from one location to another within the same site.
EP 1 736 664 discloses a method and an apparatus for controlling a wind turbine. Values of relevant measurement parameters, e.g. wind speed and/or wind direction, are measured, e.g. at an external measurement station or at a neighbouring wind turbine. The measured values are processed and supplied to a control unit. The control unit then selects a set of control parameters and the wind turbine is controlled in accordance with these control parameters.
US 2007/0183885 A1 discloses a method for optimising the operation of a wind turbine. A control parameter is adjusted to a predetermined starting value. A response variable and a further variable indicative of an ambient condition are measured. This is repeated a number of times, the control parameter being varied at each repetition. A measured relation between the control parameter and the response variable and the variable indicative of an ambient condition is determined. Based on this, an optimised value of the control parameter with respect to the response variable is determined, and a set point of the control parameter is adjusted to the optimised value.